Electromagnetic actuator

ABSTRACT

The rotational torque of an electromagnetic actuator having a stator and rotor, such as a stepping motor, is increased. 
     The electromagnetic actuator includes a rotor ( 30 ) having multiple magnetic poles (Nr 1 , Nr 2 , Nr 3 , Nr 4 , Nr 5 , Sr 1 , Sr 2 , Sr 3 , Sr 4 , Sr 5 ) on a circumferential surface ( 31 ) and a plurality of stators ( 10, 20 ) in an electromagnet form. The stator ( 10, 20 ) has a pair of magnetic poles ( 13, 14, 23, 24 ) facing the circumferential surface ( 31 ) of the rotor ( 30 ), and an auxiliary yoke ( 40, 50 ) made of a soft magnetic material member extending in a circumferential direction (A) over an angular range (γ 1, γ2 ) larger than a spread angle (α) of the magnetic poles (Nr, Sr) of the rotor ( 30 ) in the circumferential direction (A) is provided between the pair of magnetic poles ( 13, 14, 23, 24 ).

TECHNICAL FIELD

The present invention relates to an electromagnetic actuator usable as,for example, a stepping motor.

BACKGROUND ART

It is desirable that a stepping motor which is used for zoom drive orthe like of a small camera installed in a portable device should have asmall planar size and be thin. In case where it is used in a portabledevice, the drive source is often a battery and should desirably have ahigh motor efficiency and low current consumption. When control withhigh positional precision is desirable as in zoom drive of a camera, therotor of a stepping motor is made to have multipoles in such a way as toguarantee a relatively high rotational position accuracy, e.g., to beable to designate the rotational position of 20 steps/rotation or so.When a rotor of a small diameter is made to have multipoles, the widthof the magnetic poles of the stator that should be selectively made toface the magnetic poles of the rotor becomes smaller, saturation islikely to occur and leakage of the magnetic flux is likely to becomelarge. Further, miniaturization and multipolarization of the rotor makethe anisotropy of the shapes of individual electromagnet portions of therotor non-negligible, and the magnetized states of the individualelectromagnet portions of the rotor may become imperfect to call them apermanent magnet.

There has been a proposal to arrange members of a soft magnetic materialclose to the rotor in order to improve the rotational performance ofsuch a stepping motor or the stop stability in the characteristics atthe time of step rotation (Patent Literature 1, Patent Literature 2).

Patent Literature 1: Unexamined Japanese Patent Application KOKAIPublication No. 2003-32991

Patent Literature 2: Unexamined Japanese Patent Application KOKAIPublication No. 2002-136095

DISCLOSURE OF INVENTION Problem to be Solved by the Invention

However, the prior arts proposed in those Patent Literatures 1 and 2 arethe one which has members of a soft magnetic material provided withinthe range of an angle between the magnetic poles of the rotor or less(typically, about ½ of the angle) for the purpose that the rotor cansurely stop at the step rotational position of the rotor by enhancingthe magnetostatic stability at the step rotational position (PatentLiterature 1) or the one which has members of a soft magnetic materialprovided within the range of an angle between the magnetic poles of therotor or less (typically, about ½ of the angle) for the purpose that therotor is suppressed from excessively pulled to the step rotationalposition of the rotor (the detent torque is reduced) to make the rotorsmoothly rotate by reducing the magnetostatic stability at the steprotational position (Patent Literature 2), and are not intended toimprove the rotational torque of the stepping motor.

The present invention has been made in view of the aforementionedpoints, and it is an object of the invention to provide anelectromagnetic actuator capable of enhancing the rotational torque.

Means for Solving the Problem

An electromagnetic actuator according to the present invention comprisesa rotor which has multiple magnetic poles on a circumferential surfacethereof, and a plurality of stators in an electromagnet form which haveat least two magnetic poles facing the circumferential surface of therotor, wherein an auxiliary yoke extending in a circumferentialdirection of the magnetic poles of the rotor over an angular range whichis larger than a spread angle of the magnetic poles of the rotor in thecircumferential direction is provided between the magnetic poles of atleast one of the stators.

EFFECT OF THE INVENTION

The present invention can provide an electromagnetic actuator capable ofenhancing the rotational torque.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] It is a perspective explanatory diagram of a stepping motoraccording to one preferable embodiment of the present invention.

[FIG. 2] It is an exemplary plan explanatory view of the stepping motorin FIG. 1.

[FIG. 3] It illustrates the 2-phase excitation rotational movement ofthe motor in FIG. 1, and FIGS. 3A, 3B, 3C and 3D are exemplary planexplanatory views showing states Q1, Q2, Q3 and Q4 which shift in order.

[FIG. 4] It illustrates a part of the 1-2-phase excitation rotationalmovement of the motor in FIG. 1, and FIGS. 4A, 4B, 4C and 4D areexemplary plan explanatory views showing states Q1, Q12, Q2 and Q23which shift in order.

[FIG. 5] It illustrates the remaining part of the 1-2-phase excitationrotational movement of the motor in FIG. 1, and FIGS. 5A, 5B, 5C and 5Dare exemplary plan explanatory views showing states Q3, Q34, Q4 and Q41which shift in order.

[FIG. 6] It illustrates modifications (second embodiment) of thestepping motor in FIG. 1, FIG. 6A is a plan explanatory view of onemodification, FIG. 6B is a perspective explanatory view of the steppingmotor in FIG. 6A, FIG. 6C is a plan explanatory view of anothermodification, and FIG. 6D is a perspective explanatory view of thestepping motor in FIG. 6C.

[FIG. 7] It is a perspective explanatory diagram of a stepping motoraccording to another preferable embodiment (third embodiment) of theinvention.

[FIG. 8] It is an exemplary plan explanatory view of the stepping motorin FIG. 7.

[FIG. 9] It is a perspective explanatory diagram of a stepping motoraccording to a further preferable embodiment (fourth embodiment) of theinvention.

[FIG. 10] It is an exemplary plan explanatory view of the stepping motorin FIG. 9.

[FIG. 11] It is a perspective explanatory diagram of a stepping motoraccording to a yet further preferable embodiment (fifth embodiment) ofthe invention.

[FIG. 12] It is an exemplary plan explanatory view of the stepping motorin FIG. 11.

[FIG. 13] It is a graph showing a change in torque when the 2-phaseexcitation rotational movement of the stepping motor in FIG. 1 as inFIG. 3 is performed.

EXPLANATION OF REFERENCE NUMERALS

-   1, 1 a, 1 b, 1 c stepping motor-   5 members of a soft magnetic material (correlation)-   10, 10 a, 10 b stator-   11 yoke-   12 coil-   13, 14 magnetic pole-   15, 16 endface-   20, 20 a, 20 b stator-   21 yoke-   22 coil-   23, 24 magnetic pole-   25, 26 end face-   30, 30 a, 30 b rotor-   31 circumferential surface-   33 top surface-   35 rotor body-   36 rotor shaft (output shaft of motor)-   40, 40 a, 40 b, 50, 50 a, 50 b, 80, 90 auxiliary yoke (soft magnetic    material)-   50i imaginary line-   41, 42, 51, 52 circumferential-directional both end portions-   43, 53 thin portion-   44, 54 opening-   45, 55 thick portion at axial-directional both end portions-   60 stator-   61 yoke-   62 coil-   63, 64 magnetic pole-   70 stator-   71 yoke-   72 coil-   73, 74 magnetic pole-   A circumferential direction-   A1 rotational direction-   C center axial line (of rotor)-   G₁₁, G₁₂, G₂₁, G₂₂ gap (gap between stator magnetic pole and rotor    circumferential surface)-   g1, g2, g3 gap (gap between auxiliary yoke and rotor circumferential    surface)-   I1, I2 current-   Lr magnetic pole length-   Ls, Ls1, Ls2 circumferential-directional length of auxiliary yoke-   Kl, K2 closed magnetic path-   Mc mirror symmetrical center line-   Nr, Nr₁, Nr₂, Nr₃, Nr₄, Nr₅, Nr₆, Nr₇ N pole (rotor magnetic pole)-   Ql, Q12, Q2, Q23, Q3, Q34, Q4, Q41 state of motor-   Sr, Sr₁, Sr₂, Sr₃, Sr₄, Sr₅, Sr₆, Sr₇ S pole (rotor magnetic pole)-   Tr, Ts torque-   V1, V2, V3, V4 terminal-   α spread angle of rotor magnetic poles (interval between angles of    rotor magnetic poles)-   β1, β2 interval between angles of adjacent magnetic poles    (correlation) of adjacent stator-   βs₁₁, βs₁₂, βs₂₁, βs₂₂, βs_(p) spread angle of magnetic poles of    stator-   βs₁, βs₂, βs interval between angles of magnetic poles of stator-   γ, γ1, γ2, γm spread angle of auxiliary yoke-   κ₁, κ₂ magnetic path between auxiliary yoke and rotor

An electromagnetic actuator according to an embodiment of the presentinvention comprises a rotor which has multiple magnetic poles on acircumferential surface thereof, and a plurality of stators in anelectromagnet form which have at least two magnetic poles facing thecircumferential surface of the rotor, and an auxiliary yoke extending ina circumferential direction of the magnetic poles of the rotor over anangular range which is larger than a spread angle of the magnetic polesof the rotor in the circumferential direction is provided between themagnetic poles of at least one of the stators.

In the electromagnetic actuator according to the embodiment of thepresent invention, as the auxiliary yoke extending in a circumferentialdirection of the magnetic poles of the rotor over an angular range whichis larger than a spread angle of the magnetic poles of the rotor in thecircumferential direction is provided between the magnetic poles of thestator, the auxiliary yoke so faces the circumferential surface of therotor as to step over (yoke) the adjoining magnetic poles of the rotor,making it possible to provide a magnetostatic path which yokes theadjoining magnetic poles. Accordingly, as at least a portion of themagnetostatic path which extends from one magnetic pole of the stator tothe other magnetic pole of the stator via the rotor is given by theauxiliary yoke (generally made of a soft magnetic material), theintensity of a magnetic field between a gap (magnetic gap) of the frontsurface of the magnetic poles of the stator can be enhanced, whereby therotational torque of the rotor, that is, the rotational performance canbe improved.

As the auxiliary yoke provides the magnetostatic path between theadjoining magnetic poles of the rotor, the magnetostatical connection ofthe magnet portions of the rotor whose magnetic poles face the auxiliaryyoke contributes to the improvement of the performance.

The rotor is typically magnetized in the radial direction. However, fromthe viewpoint of the magnetizing technology, the direction anddistribution of the magnetization of the inside close to the center maydiffer from the radial direction. The magnetic poles of the rotortypically spread over the same length or the same angular range as seenin the circumferential direction, and gaps or angles between theadjoining magnetic poles in the circumferential direction are also thesame. In some cases, however, both may differ to some extent. In otherwords, the rotor typically has even magnetic poles, that is, italternately has the same numbers of the N poles and the S poles, and thespread or the angles of the individual magnetic poles in thecircumferential direction are the same, but a slight difference isacceptable. It is preferable that the number of the magnetic poles onthe circumferential surface of the rotor be, typically, 10 to 14 polesor so, but it may be less or larger than that in some cases.

The rotor is a permanent magnet type having the magnetic poles on itscircumferential surface, and is made of a permanent magnet material, inparticular, a permanent magnet with large retentive power, and ingeneral, one so-called a hard magnetic material is suitable. However, asthe aspect ratio (the length of the direction of the magnetization/thewidth (the length in the orthogonal direction)) of the magnetized areacorresponding to each surface magnetic pole of the rotor is not alwayslarge, the sufficient magnetization may be difficult because of theinfluence of the demagnetization which depends on the shape of eachmagnetized area, in which case the intensity or the magnetic fluxdensity of the magnetic pole at the circumferential surface of the rotorcan be enhanced by connecting one magnetized area to the othermagnetized area in a magnetostatic manner.

The stator is in the form of an electromagnet including a coil forexcitation, and comprises a yoke which is made of a soft magneticmaterial, typically a magnetic material with high permeability and largesaturation magnetization like Permalloy, and an excitation coil whichmagnetizes this yoke. The yoke is formed in a letter “U” or “C” shape,or a similar shape having at least two portions which become themagnetic poles when excited. The magnetic pole portions of the statorare so arranged as to face the circumferential surface of the rotor, viaa narrow gap or magnetic-pole-clearance (magnetic pole gap). In thisspecification, the direction of a pair of foot of “U” itself is not tobe specified, and unless otherwise expressly or contextually specified,the “U” shape and the “C” shape are treated as the same shape. In somecases, branches may be so formed as to have, for example, a letter “E”shape as a whole. The magnetic poles of the stator typically spread overhalf the angular range of the spread angle of the individual magneticpoles of the rotor. In some cases, however, it may be smaller or largerdepending on how to carry out the excitation rotation as long as it isequal to or smaller than the spread angle of the magnetic poles of therotor.

For instance, when the auxiliary yoke of about 3/2 of the spread angleof each magnetic pole of the rotor is placed between the magnetic polesof the stator, and the spread angles of the magnetic poles of the statorare about ½ of the spread angle of each magnetic pole of the rotor, agap between the pair of magnetic poles of the stator is typically largerthan or equal to 5/2 times the spread angle of the magnetic poles of therotor, matching the magnetic pole gap of the rotor. That is, when theauxiliary yoke is placed between the magnetic poles of the stator, anangular gap between the magnetic pole of the stator and the adjoiningend portion of the auxiliary yoke is typically larger than or equal to ½times the spread angle of the magnetic pole of the rotor. As long as themagnetic poles of the stator are not and practically directly connectedto the adjoining end portions of the auxiliary yoke in a magnetostaticmanner, the angular gap between the magnetic poles of the stator and theadjoining end portions of the auxiliary yoke may be small. In otherwords, as long as a magnetic flux, which flows between the magneticpoles of the stator and the adjoining end portions of the auxiliaryyoke, is small enough to be negligible as compared with a magnetic fluxwhich flows between the magnetic poles of the stator and thecircumferential surface of the rotor, the angular gap between themagnetic poles of the stator and the adjoining end portions of theauxiliary yoke may be smaller.

In an electromagnetic actuator like a stepping motor, two or morestators are provided. As long as the adjoining stators in thecircumferential direction are substantially separated from each other ina magnetostatic manner without intervention of the rotor and theauxiliary yoke, the angular gap between the mutually adjoining magneticpoles of the stators adjoining in the circumferential direction may besubstantially equal to 0.

As a way of driving the stator with respect to the excitation coil,2-phase excitation rotational drive or 1-2-phase excitation rotationaldrive is typically adapted. Other different excitation rotationalmovement may be carried out.

Typically, the auxiliary yoke is provided between a pair of magneticpoles of each stator. The auxiliary yoke may be provided between themagnetic poles of one stator among the plurality of stators.

As long as the auxiliary yoke substantially extends in thecircumferential direction over the angular range larger than the spreadangle of the. magnetic poles of the rotor in the circumferentialdirection to thereby face both of different magnetic poles (N pole and Spole) on the circumferential surface of the rotor, it may be almostequal to the spread angle of the magnetic pole of the rotor in thecircumferential direction, or larger than the spread angle of themagnetic pole of the rotor in the circumferential direction. However, inorder to certainly provide a magnetostatic path between differentmagnetic poles of the rotor which adjoin in the circumferentialdirection, it is preferable that the auxiliary yoke be larger than orequal to 3/2 times the spread angle of the magnetic poles of the rotorin the circumferential direction. To suppress the angle between a pairof magnetic poles of the stator to a certain range, it is preferablethat the auxiliary yoke should not be excessively large with respect tothe spread angle of the magnetic poles of the rotor in thecircumferential direction.

As mentioned in the above explanation for the arrangement of the stator,it is necessary to avoid the direct and magnetostatic contact of thestator and the auxiliary yoke, and, in order to do so, the end portionsof the auxiliary yoke in the circumferential direction should be awayfrom the magnetic poles of the stator by a certain distance, andpreferably, it should be kept as apart by ½ of the spread angle of themagnetic pole of the rotor in the circumferential direction or greater.To minimize the magnet resistance of the magnetostatic path between apair of magnetic poles of the stator at a portion other than the frontmagnetic gap portions of the magnetic poles of the stator, however, itis preferable that the end portions of the auxiliary yoke be in a closeproximity to the adjoining magnetic poles of the stator as long as theauxiliary yoke do not directly contact the stator in a magnetostaticmanner. The end portions of the stator are arranged at a gap of about ½of the spread angle of the magnetic poles of the rotor in thecircumferential direction with respect to the adjoining magnetic polesof the stator. In some cases, as long as it is possible to avoidsubstantial formation of the short-circuited magnetostatic path betweenthe auxiliary yoke and the stator, the auxiliary yoke and the stator maybe integrally connected with each other via a portion with a small crosssectional area (this portion is to be magnetically saturated) to allowthe auxiliary yoke and the stator, which are integrated partsmechanically, to function as if they were magnetically separated.

The auxiliary yoke is arranged in a close proximity to thecircumferential surface of the rotor via a gap with substantively thesame size as the gap (magnetic gap) between the magnetic poles of thestator and the circumferential surface of the rotor, so that it can bemagnetostatically connected between the circumferential surface of therotor and the areas facing the circumferential surface of the rotor. Theauxiliary yoke, however, may be so provided as to have a small gapbetween the rotor and one portion facing one magnetic pole, a small gapbetween the rotor and another portion facing another adjoining magneticpole (S pole or N pole), and a connecting portion or joining portionhaving a low magnetic resistance between one portion and anotherportion, so that a magnetostatic path extending in the circumferentialdirection can be provided between one magnetic pole (N pole or S pole)of the rotor facing the auxiliary yoke, and another adjoining magneticpole (S pole or N pole) different from the one magnetic pole. In thiscase, the auxiliary yoke may be substantially apart from thecircumferential surface of the rotor at other portions than twoportions, namely, one portion of the auxiliary yoke and another portionthereof. For instance, there may be a groove or a cutaway, extending inparallel to the axial direction between both end portions of theauxiliary yoke in the circumferential direction, or there may be anopening between both end portions of the auxiliary yoke in thecircumferential direction. The auxiliary yoke may have a largeraxial-directional length or nearly the same axial-directional length ascompared with the axial-directional thickness of the disk-like rotor,and it may have a shorter axial-directional length than theaxial-directional thickness of the rotor as long as it is stronglymagnetostatically connected to the magnetic pole of the circumferentialsurface.

The auxiliary yoke is made of a soft magnetic material so as to providea magnetostatic path with a low magnet resistance, and preferably, ahigh-permeability material should be used.

Next, preferable embodiments of the present invention will now beexplained with reference to the accompanying drawings.

First Embodiment

A stepping motor 1 of the first embodiment according to the presentinvention is illustrated in FIGS. 1 and 2. The stepping motor 1comprises two sets of stators 10 and 20, a rotor 30, and auxiliarystators or auxiliary yokes 40, 50 which are made of a soft magneticmaterial.

The rotor 30 has an approximately disk-like or annular plate shape. Itcomprises a permanent magnet structure in which ten magnetic polesmagnetized on its circumferential surface 31 at substantively equalintervals in the radial direction. In FIG. 2, the circumferentialsurfaces of areas Nr₁ to Nr₅ (denoted by reference numeral Nr when theyare not mutually distinguished or generically referred to) are somagnetized as to be N poles, and the circumferential surfaces of theremaining areas Sr₁ to Sr₅ (denoted by reference numeral Sr when theyare not mutually distinguished or generically referred to) are somagnetized as to be S poles. In the exemplary diagram of FIG. 2,however, the purpose is to easily illustrate that N and S polesalternately appear in the circumferential surface 31 at substantivelyequal intervals, i.e., to distinguish the magnetized states on thecircumferential surface 31 and in the vicinity thereof, but thisexemplary diagram does not strictly define the internal magnetizedstates in the radial direction. As illustrated in the perspective viewof FIG. 1, the rotor 30 typically comprises a rotor body 35 of anannular plate shape, and a rotor shaft (motor output shaft) 36 overwhich the rotor body 35 of the annular plate shape is fitted. The rotorbody may be formed integral with the rotor shaft.

The stator 10 comprises a yoke 11 made of a soft magnetic material of a“U” shape or a “C” shape, and a stator coil 12, and magnetic poles 13and 14 at both ends of the yoke 11 are position in a close proximity tothe circumferential surface 31 of the rotor 30 via narrow gaps G₁₁ andG₁₂. The spread angle of front end surfaces 15 and 16 of the magneticpoles 13 and 14 along the circumferential direction A (the angle viewedfrom the center of the rotor) are βs_(p) (βs₁₁, βs₁₂), and aresubstantively ½ of the spread angle α of one of the magnetic poles N andS, of the rotor 30. The open angle βs₁, between the magnetic poles 13and 14 in the circumferential direction A is 5/2 of the spread angle αof the magnetic pole of the rotor 30, and two magnetic poles 13 and 14face the opposite magnetic poles Nr and Sr, or Sr and Nr of the rotor30, regardless of the rotational position of the rotor 30.

As the stator 20 is structured in the same way as the stator 10, itsdetailed explanation will be omitted.

The spread angle β₁ between the magnetic pole 13 of the stator 10 andthe magnetic pole 23 of the stator 20 in the circumferential direction Ais twice as much as the spread angle α of the rotor 30 (the relativeangular gap of the magnetic poles 13 and 23 is 5α/2). The gap β₂ betweenthe magnetic pole 14 of the stator 10 and the magnetic pole 24 of thestator 20 in the circumferential direction A is one times the spreadangle α of the rotor 30 (the relative angular gap of the magnetic poles14, 24 is 3α/2). Accordingly, in the 2-phase excitation rotationalmovement to be discussed later, for example, two magnetic poles 13 and14 repeat facing the opposite magnetic poles Nr and Sr or Sr and Nr ofthe rotor 30 every time the rotor 30 makes 1/20 rotation twice, and twomagnetic poles 23 and 24 also repeat facing the opposite magnetic polesNr and Sr or Sr and Nr of the rotor 30 every time the rotor 30 makes1/20 rotation twice.

The auxiliary yoke 40 comprises a partly cylindrical body, and faces thecircumferential surface 31 of the rotor 30 via a gap g1 between themagnetic poles 13 and 14 of the stator 10. The spread angle γ (γ1) ofthe auxiliary yoke 40 in the circumferential direction A is 3/2 timesthe spread angle α of the magnetic poles Nr and Sr of the rotor 30, inother words, the length Ls (Ls1) of the auxiliary yoke 40 in thecircumferential direction A is 3/2 times the magnetic pole length Lr ofthe rotor 30, provided that the gap g1 is of a negligible size.

Accordingly, the auxiliary yoke 40 faces two magnetic poles Nr and Sr,adjoining in the circumferential direction A of the rotor 30, in theradial direction at an arbitrary point of time. As a result, theauxiliary yoke 40 is formed with a path of a magnetic flux(magnetostatic path) κ1 which enters the auxiliary yoke 40 from themagnetic pole Nr facing the rotor 30, passes through the inside of theauxiliary yoke 40, and then returns to the magnetic pole Sr of the rotor30 facing the auxiliary yoke 40 from the auxiliary yoke 40.

The auxiliary yoke 40 is so arranged as to be at the equal intervalsfrom the magnetic poles 13 and 14 of the stator 10, and in thisembodiment, it is separated from the magnetic poles 13 and 14 of thestator 10 by the angle of γ/2 (the circumferential distance of Ls/2).

The auxiliary yoke 50 has the same structure as the auxiliary yoke 40,and thus its detailed explanation will be omitted.

The operation when the stepping motor 1 constructed in the above mannertakes a 2-phase excitation rotation will be described based on FIG. 3.

FIG. 3A shows a state Q1 where terminals V1 and V2 of the stator 10 arerespectively set to positive and negative, and a current I1 is let toflow in the coil 12 so that the magnetic poles 13 and 14 are somagnetized as to be the N pole and S pole, respectively, while terminalsV3 and V4 of the stator 20 are respectively set to negative andpositive, and a current I2 is let to flow in the coil 22 so that themagnetic poles 23 and 24 are respectively magnetized to the S pole and Npole.

In the state Q1 in FIG. 3A, the first half portion of the magnetic poleSr₁ of the rotor 30 in the A1 direction and the first half portion ofthe magnetic pole Nr₂ in the A1 direction, and the second half portionof the magnetic pole Nr₄ of the rotor 30 in the A1 direction and thesecond half portion of the magnetic pole Sr₃ in the A1 direction areattracted to the facing magnetic poles of the stators 10 and 20 in sucha state where the first half portion of the magnetic pole Sr₁ and thefirst half portion of the magnetic pole Nr₂ respectively face themagnetic pole 13 magnetized to the N pole and the magnetic pole 14magnetized to the S pole in the stator 10, and the second half portionof the magnetic pole Nr₄ and the second half portion of the magneticpole Sr₃ respectively face the magnetic pole 23 magnetized to the S poleand the magnetic pole 24 magnetized to the N pole in the stator 20.Although the following description is given on the assumption that themagnetic pole Sr₁ is facing the magnetic pole 13 of the stator 10, it isapparent that the same is true of any of the other magnetic poles Sr₂,Sr₃, Sr₄ and Sr₅ in place of the magnetic pole Sr₁ due to the rotationalsymmetry.

In the state Q1, the entire magnetic pole Nr₁ and a part of the magneticpole Sr₂ of the rotor 30 face the auxiliary yoke 40, and a magnetic pathκ1 which leaves the magnetic pole Nr₁ of the rotor 30, passes theauxiliary yoke 40 and returns to the magnetic pole Sr₂ is formed betweenthe rotor 30 and the auxiliary yoke 40. Regarding the stator 10,therefore, a substantial portion of the magnetic flux of a closedmagnetic path K1 which enters the facing magnetic pole Sr₁ of the rotorfrom the magnetic pole (N pole) 13 through a gap G₁₁, passes through themagnetic pole Nr₂ of the rotor, then returns to the magnetic pole (Spole) 14 of the stator 10 through a gap G₁₂ is given by the auxiliaryyoke 40 of a soft magnetic material. As compared with a case where thereis no auxiliary yoke 40, therefore, the magnetic resistance of theclosed magnetic path K1 is reduced considerably. As a result, theintensities of the magnetic fields at the front gaps G₁₁ and G₁₂ of themagnetic poles 13 and 14 of the stator 10 according to the current I1are enhanced, thereby increasing the magnetostatic attraction withrespect to the magnetic poles Sr₁ and Nr₂ of the rotor 30.

Likewise, in the state Q1, a part of the magnetic pole Nr₃ and theentire magnetic pole Sr₄ of the rotor 30 face the auxiliary yoke 50, anda magnetic path κ2 which leaves the magnetic pole Nr₃ of the rotor 30,passes the auxiliary yoke 50 and returns to the magnetic pole Sr₄ isformed between the rotor 30 and the auxiliary yoke 50. Regarding thestator 20, therefore, a substantial portion of the magnetic flux of aclosed magnetic path K2 which enters the facing magnetic pole Sr₃ of therotor from the magnetic pole (N pole) 24 through a gap G₂₂, passesthrough the magnetic pole Nr₄ of the rotor 30, then returns to themagnetic pole (S pole) 23 of the stator 20 is given by the auxiliaryyoke 50 of a soft magnetic material. As compared with a case where thereis no auxiliary yoke 50, therefore, the magnetic resistance of theclosed magnetic path K2 is reduced considerably. As a result, theintensities of the magnetic fields at the front gaps G₂₂ and G₂₁ of themagnetic poles 24 and 23 according to the current I2 are increased, sothat the magnetostatic attraction with respect to the magnetic poles Sr₃and Nr₄ of the rotor 30 can be increased.

The increase in magnetostatic attraction can enhance the magnetostaticstability of the rotor 30 of the stepping motor 1 in the state Q1. Inother words, it also means that the rotor 30 is kept stopped surely atthe rotational position in the state Q1.

At the next step, as shown in FIG. 3B, with the state of the stator 10left unchanged, the positive and negative states of the terminals V3 andV4 of the coil 22 of the stator 20 are inverted to be positive andnegative, respectively, and a current −I2 is let to flow in the coil 22to bring about a state Q2 where the magnetic poles 23 and 24 are somagnetized as to be the N pole and the S pole, respectively.

In the state Q2, the magnetic poles 23 and 24 respectively change to theN pole and the S pole. The magnetic pole Sr₅ of the rotor 30 isattracted to the magnetic pole 23, and the magnetic pole Nr₃ of therotor is attracted to the magnetic pole 24, so that the rotor 30 rotatesby 1/20 in the A1 direction and takes a position as shown in FIG. 3B.

In the state Q2, a part of the magnetic pole Nr₁ and the entire magneticpole Sr₂ of the rotor 30 face the auxiliary yoke 40, and the magneticpath κ1 which leaves the magnetic pole Nr₁ of the rotor 30, passes theauxiliary yoke 40 and returns to the magnetic pole Sr₂ is formed betweenthe rotor 30 and the auxiliary yoke 40. Regarding the stator 10,therefore, a substantial portion of the magnetic flux of the closedmagnetic path K1 which enters the facing magnetic pole Sr₁ of the rotor30 from the magnetic pole (N pole) 13 through the G₁₁, passes throughthe magnetic pole Nr₂ of the rotor, then returns to the magnetic pole (Spole) 14 of the stator 10 through the gap G₁₂ is given by the auxiliaryyoke 40 of a soft magnetic material. As compared with a case where thereis no auxiliary yoke 40, therefore, the magnetic resistance of theclosed magnetic path K1 can be reduced considerably. As a result, theintensities of the magnetic fields at the front gaps G₁₁ and G₁₂ of themagnetic poles 13 and 14 of the stator 10 according to the statorcurrent I1 are enhanced, so that the magnetostatic attraction withrespect to the magnetic poles Sr₁ and Nr₂ of the rotor 30 can beincreased. The action of the magnet portion facing the auxiliary yoke 40of the rotor 30 is the same as the one mentioned above.

Likewise, in the state Q2, a part of the magnetic pole Nr₃ and theentire magnetic pole Sr₄ of the rotor 30 face the auxiliary yoke 50, andthe magnetic path κ2 which leaves the magnetic pole Nr₄ of the rotor 30,passes the auxiliary yoke 50 and returns to the magnetic pole Sr₄ isformed between the rotor 30 and the auxiliary yoke 50. Regarding thestator 20, therefore, a substantial portion of the magnetic flux of theclosed magnetic path K2 which enters the facing magnetic pole Sr₅ of therotor from the magnetic pole (N pole) 23 through the gap G₂₁, passesthrough the magnetic pole Nr₃ of the rotor, then returns to the magneticpole (S pole) 24 of the stator 20 is given by the auxiliary yoke 50 of asoft magnetic material. As compared with a case where there is noauxiliary yoke 50, therefore, the magnetic resistance of the closedmagnetic path K2 can be reduced considerably. As a result, theintensities of the magnetic fields at the front gaps G₂₁ and G₂₂ of themagnetic poles 23 and 24 of the stator 20 according to the statorcurrent −I2 are increased, so that the magnetostatic attraction withrespect to the magnetic poles Sr₅ and Nr₃ of the rotor 30 can beincreased. The action of the magnet portion facing the auxiliary yoke 50of the rotor 30 is the same as the one mentioned above.

Such an increase in magnetostatic attraction can enhance themagnetostatic stability of the rotor 30 of the stepping motor 1 in thestate Q2. In other words, it also means that the rotor 30 is keptstopped surely at the rotational position in the state Q2.

Paying attention to the rotation of the rotor 30 from the state Q1 tothe state Q2, the rotational torque is increased by the intensities ofthe magnetic fields at the magnetic gaps (clearances) G₁₁, G₁₂, G₂₁ andG₂₂ caused by reduction in magnetic resistances of the closed magneticpaths K1 and K2 by the provision of the auxiliary yokes 40 and 50.

At the next step, as shown in FIG. 3C, with the state of the stator 20left unchanged, the positive and negative states of the terminals V1 andV2 of the coil 12 of the stator 20 are inverted to be negative andpositive, respectively, and a current −I1 is let to flow in the coil 12to bring about a state Q3 where the magnetic poles 13 and 14 are somagnetized as to be the S pole and the N pole, respectively.

In the state Q3, the magnetic pole Sr₃ of the rotor 30 is attracted tothe magnetic pole 14 which has changed to the N pole, and the magneticpole Nr₁ of the rotor is attracted to the magnetic pole 13 which haschanged to the S pole, so that the rotor 30 rotates by 1/20 in the A1direction and takes a position as shown in FIG. 3C.

In the state Q3, a part of the magnetic pole Nr₂ and the entire magneticpole Sr₂ of the rotor 30 face the auxiliary yoke 40, and the magneticpath κ1 which leaves the magnetic pole Nr₂ of the rotor 30, passes theauxiliary yoke 40 and returns to the magnetic pole Sr₂ is formed betweenthe rotor 30 and the auxiliary yoke 40. Regarding the stator (magnet)10, therefore, a substantial portion of the closed magnetic path K1which enters the facing magnetic pole Sr₃ of the rotor magnet 30 fromthe magnetic pole (N pole) 14 through the G₁₂, passes through themagnetic pole Nr₁ of the rotor magnet 30, then returns to the magneticpole (S pole) 13 of the stator (magnet) 10 through the gap G₁₁ is givenby the auxiliary yoke 40 of a soft magnetic material. As compared with acase where there is no auxiliary yoke 40, therefore, the magneticresistance of the closed magnetic path K1 can be reduced considerably.As a result, the intensities of the magnetic fields at the front gapsG₁₁ and G₁₂ of the magnetic poles 13 and 14 of the stator 10 accordingto the stator current −I1 are enhanced, so that the magnetostaticattraction with respect to the magnetic poles Nr₁ and Sr₃ of the rotor30 can be increased.

Likewise, in the state Q3, a part of the magnetic pole Sr₃ and theentire magnetic pole Nr₄ of the rotor 30 face the auxiliary yoke 50, andthe magnetic path κ2 which leaves the magnetic pole Nr₄ of the rotor 30,passes the auxiliary yoke 50 and returns to the magnetic pole Sr₄ isformed between the rotor 30 and the auxiliary yoke 50. Regarding thestator (magnet) 20, therefore, a substantial portion of the closedmagnetic path K2 which enters the facing magnetic pole Sr₅ of the rotor(magnet) 30 from the magnetic pole (N pole) 23 through the gap G₂₁,passes through the magnetic pole Nr₃ of the rotor (magnet) 30, thenreturns to the magnetic pole (S pole) 24 of the stator (magnet) 20 isgiven by the auxiliary yoke 50 of a soft magnetic material. As comparedwith a case where there is no auxiliary yoke 50, therefore, the magneticresistance of the closed magnetic path K2 can be reduced considerably.As a result, the intensities of the magnetic fields at the front gapsG₂₁ and G₂₂ of the magnetic poles 23 and 24 of the stator 20 accordingto the stator current −I2 are increased, so that the magnetostaticattraction with respect to the magnetic poles Sr₅ and Nr₃ of the rotor30 can be increased.

Such an increase in magnetostatic attraction can enhance themagnetostatic stability of the rotor 30 of the stepping motor 1 in thestate Q3. In other words, it also means that the rotor 30 is keptstopped surely at the rotational position in the state Q3.

Meanwhile, paying attention to the rotation of the rotor 30 from thestate Q2 to the state Q3, the rotational torque is increased by theintensities of the magnetic fields at the magnetic gaps (clearances)G₁₁, G₁₂, G₂₁ and G₂₂ caused by reduction in magnetic resistances of theclosed magnetic paths K1 and K2 by the provision of the auxiliary yokes40 and 50.

Likewise, at the next step, as shown in FIG. 3D, with the state of thestator 10 left unchanged, the positive and negative states of theterminals V3 and V4 of the coil 22 of the stator 20 are inverted to benegative and positive, respectively, and the current I2 is let to flowin the coil 22 to bring about a state Q4 where the magnetic poles 23 and24 of the stator 20 are so magnetized as to be the S pole and the Npole, respectively. Accordingly, the rotor 30 is further rotated by1/20. Even in this case, both of the rotational torque in the A1direction from the state Q3 to the state Q4 and the magnetostaticstability of the state Q4 can be enhanced because of the same reason asmentioned above.

Further, at the next step, as shown in FIG. 3A, with the state of thestator 20 left unchanged, the positive and negative states of theterminals V1 and V2 of the stator 10 are inverted to be positive andnegative, respectively, and the current I1 is let to flow in the coil 12to bring about the state Q1 where the magnetic poles 13 and 14 are somagnetized as to be the S pole and the N pole, respectively.Accordingly, the rotor 30 is further rotated by 1/20. Even in this case,both of the rotational torque in the A1 direction from the state Q4 tothe state Q1 and the magnetostatic stability of the state Q1 can beenhanced because of the same reason as mentioned above.

Thereafter, the states Q2, Q3, Q4 and Q1 are repeated cyclically and therotor 30 is rotated in the A1 direction. Accordingly, both therotational torque and the magnetostatic stability at the step rotationalposition of the motor 1 having the auxiliary yokes 40 and 50 areimproved.

TEST EXAMPLES

For the stepping motor 1 shown in FIGS. 1 and 2, a case where the2-phase excitation rotational movement is made as shown in FIG. 3 wassimulated using magnetic field analysis. The simulation conditions andresults are as follows.

Simulation Conditions

1. About Rotor 30

(1) Shape: Annular Shape as Shown in FIG. 1

Outside diameter: 3 mm, inside diameter: 0.6 mm, thickness: 1.2 mm(equivalent to the aspect ratio of 1.3 to 6.4 or so on the assumptionthat a magnetized area is formed in the radial direction)

(2) Magnetic Conditions

Adapted material: SmFeN-based bonded magnet

Maximum energy product: (BH)max=111.4 kJ/m³

Magnetization direction: radial direction

Magnetization distribution (magnetic flux density distribution at thecircumferential surface 31): sin 5θ (θ is an angle with the center ofthe magnetic pole area as the origin)

2. About Stators 10 and 20

(1) Magnetic Conditions of Yokes 11 and 21

Adapted material: 45 permalloy

Relative permeability: μm=45000

Coercive force: Hc=10 A/m

Flux density B at a magnetic field of 1000 A/m=1.4 T

(2) Excitation Conditions of Coils 12 and 22

Coil excitation at each phase: 55 AT

3. About Auxiliary Yokes 40 and 50

(1) Magnetic Conditions

Adapted material: 45 permalloy

Relative permeability: μm=45000

Antimagnetic field: Hc=10 A/m

Flux density B at a magnetic field of 1000 A/m=1.4 T

4. About the General Outer Shape

Width (X-directional length): about 10 mm, depth (Y-directional length):about 6.6 mm, height: about 2.7 mm

Simulation Results

1. The torque at the time the rotor 30 rotates counterclockwise by 36degrees from the state Q1 in FIG. 3A to the state Q3 in FIG. 3C isacquired for both of the motor 1 (simulation model) as shown in FIGS. 1and 2 and the conventional motor (Comparative Example) that is the motor1 with the auxiliary yokes 40 and 50 removed therefrom. At individualstates, a torque Ts for the simulation model is shown in FIG. 13 with atorque Tr for the motor of the Comparative Example taken as a reference(relative value of 1).2. As apparent from FIG. 13, the torque of the motor 1 of the testexample equipped with the auxiliary yokes 40 and 50 is increased byabout 1.3 to 1.7 times the torque of the conventional motor which doesnot have the auxiliary yokes.

Regarding FIG. 3, an example where 2-phase excitation rotation is madehas been described. Of course, 1-2-phase excitation may be carried outinstead as shown in FIGS. 4 and 5. In FIG. 4A, the rotational positionof the rotor 30 are the same as that in FIG. 3A.

In FIG. 4A, the motor 1 is in the state Q1 quite the same as that inFIG. 3A. Therefore, the description about FIG. 3A is directly applied tothe motor 1 in the state Q1 in FIG. 4A.

Next, as shown in FIG. 4B, energization to the coil 22 of the stator 20is stopped to cancel the excitation of the stator 20. In this state, thecenter portion of the magnetic pole Sr₁ of the rotor 30 which has largemagnetization directly faces the magnetic pole (N pole) 13 of the stator10 in the radial direction, and the center portion of the magnetic poleNr₂ of the rotor 30 which has large magnetization directly faces themagnetic pole (S pole) 14 of the stator 10 in the radial direction. Therotor 30 takes a state Q12 where it is rotated from the state Q1 by 1/40in the A1 direction. At this time, the magnetic poles 23 and 24 at bothends of the stator 20 directly face the boundary portions of the N polesand the S poles of the rotor 30, respectively.

About ⅔ of the magnetic pole Nr₁ and about ⅔ of the magnetic pole Sr₂ ofthe rotor 30 face the auxiliary yoke 40, and a magnetic path κ1 whichleaves the magnetic pole Nr₁ of the rotor 30, passes the auxiliary yoke40 and returns to the magnetic pole Sr₂ is formed. Regarding the stator10, therefore, a substantial portion of the magnetic flux of a closedmagnetic path K1 which enters the facing magnetic pole Sr₁ of the rotorfrom the magnetic pole (N pole) 13 through a gap G₁₁, passes through themagnetic pole Nr₂ of the rotor, then returns to the magnetic pole (Spole) 14 of the stator 10 through a gap G₁₂ is given by the auxiliaryyoke 40 of a soft magnetic material. As compared with a case where thereis no auxiliary yoke 40, therefore, the magnetic resistance of theclosed magnetic path K1 is reduced considerably. As a result, theintensities of the magnetic fields at the front gaps G₁₁ and G₁₂ of themagnetic poles 13 and 14 of the stator 10 according to the statorcurrent I1 are enhanced, so that the magnetostatic attraction withrespect to the magnetic poles Sr₁ and Nr₂ of the rotor 30 can beincreased.

Meanwhile, in the state Q12, about ¼ of the magnetic pole Nr₄, about ¼of the magnetic pole Nr₃ and the magnetic pole Sr₄ of the rotor 30 facethe auxiliary yoke 50.

The aforementioned increase in magnetostatic attraction between thestator 10 and the rotor 30 can enhance the magnetostatic stability ofthe rotor 30 of the stepping motor 1 in the state Q12. In other words,it also means that the rotor 30 can be kept stopped surely at therotational position in the state Q12.

Meanwhile, paying attention to the rotation of the rotor 30 from thestate Q1 to the state Q12, the rotational torque is increased by theintensities of the magnetic fields at the magnetic gaps (clearances) G₁₁and G₁₂ caused by reduction in magnetic resistance of the closedmagnetic path K1 by the provision of the auxiliary yoke 40.

At the next step, as shown in FIG. 4C, with the state of the stator 10left unchanged, the positive and negative states of the terminals V3 andV4 of the coil 22 of the stator 20 are inverted to be positive andnegative, respectively, and a current −I2 is let to flow in the coil 22to bring about a state Q2 where the magnetic poles 23 and 24 are somagnetized as to be the N pole and the S pole, respectively. This stateis the same as the state Q2 in FIG. 3B and the description of thediagram is directly applied thereto.

Paying attention to the rotation of the rotor 30 by 1/40 from the stateQ12 to the state Q2, the rotational torque is increased by theintensities of the magnetic fields at the magnetic gaps (clearances)G₁₁, G₁₂, G₂₁ and G₂₂ caused by reduction in magnetic resistances of theclosed magnetic paths K1 and K2 by the provision of the auxiliary yokes40 and 50.

Next, as shown in FIG. 4D, energization to the coil 12 is stopped tocancel the excitation of the stator 10. In this state, the centerportion of the magnetic pole Sr₅ of the rotor 30 which has largemagnetization substantially directly faces the magnetic pole (N pole) 23of the stator 20 in the radial direction, and the center portion of themagnetic pole Nr₃ of the rotor 30 which has large magnetizationsubstantially directly faces the magnetic pole (S pole) 24 of the stator20 in the radial direction. The rotor 30 takes a state Q23 where it isrotated from the state Q2 by 1/40 in the A1 direction. At this time, themagnetic poles 13 and 14 at both ends of the stator 10 which has beendeexcited directly face the boundary portions of the N poles and the Spoles of the rotor 30, respectively.

In this state Q23, about ⅔ of the magnetic pole Nr₄ and about ⅔ of themagnetic pole Sr₄ of the rotor 30 face the auxiliary yoke 50, and themagnetic path κ2 which leaves the magnetic pole Nr₄ of the rotor 30,passes the auxiliary yoke 50 and returns to the magnetic pole Sr₂ isformed. Regarding the stator 20, therefore, a substantial portion of theclosed magnetic path K2 which enters the facing magnetic pole Sr₅ of therotor magnet from the magnetic pole (N pole) 23 through the gap G₂₁,passes through the magnetic pole Nr₃ of the rotor magnet, then returnsto the magnetic pole (S pole) 24 of the stator magnet 20 through the gapG₂₂ is given by the auxiliary yoke 50 made of a soft magnetic materialand having a high permeability. As compared with a case where there isno auxiliary yoke 50, therefore, the magnetic resistance of the closedmagnetic path K2 is reduced considerably. As a result, the intensitiesof the magnetic fields at the front gaps G₂₁ and G₂₂ of the magneticpoles 23 and 24 of the stator 20 according to the stator current −I2 areenhanced, so that the magnetostatic attraction with respect to themagnetic poles Sr₅ and Nr₃ of the rotor 30 can be increased.

Meanwhile, in the state Q23, about ¼ of the magnetic pole Nr₁, about ¼of the magnetic pole Nr₂ and the magnetic pole Sr₂ of the rotor 30 facethe auxiliary yoke 40.

The aforementioned increase in magnetostatic attraction between thestator 20 and the rotor 30 can enhance the magnetostatic stability ofthe rotor 30 of the stepping motor 1 in the state Q23. In other words,it also means that the rotor 30 can be kept stopped surely at therotational position in the state Q23.

Paying attention to the rotation of the rotor 30 from the state Q2 tothe state Q23, the rotational torque is increased by the intensities ofthe magnetic fields at the magnetic gaps (clearances) G₂₁ and G₂₂ causedby reduction in magnetic resistance of the closed magnetic path K2 bythe provision of the auxiliary yoke 50.

Next, as shown in FIG. 5A, as the current −I1 is let to flow between theterminals V1 and V2 of the coil 12 in such a way that the magnetic poles14 and 13 respectively become the N and S poles with respect to thestator 10 while the excitation state of the stator 20 is maintained, thestate becomes the same state Q3 as shown in FIG. 3C. This state Q3 iswhat has been described about FIG. 3C, and the transition to the stateQ3 from the state Q23 is substantially the same as what has beendescribed about FIG. 4C.

Next, as shown in FIG. 5B, energization to the coil 22 is stopped tocancel the excitation of the stator 20 to go to a state Q34. This stateQ34 is the same as the state Q12 shown in FIG. 4B except for thedifference in the polarities of the stator 10 and the rotor 30.Therefore, the magnetostatic stability is enhanced so that therotational torque from the state Q3 to the state Q34 can be increased inthe same way as has been described for the state Q12.

Further next, as the current I2 is let to flow to the coil 22 of thestator 20 while keeping the excitation state of the stator 10, the statebecomes the same state Q4 as shown in FIG. 3D. In this state Q4, themagnetostatic stability is enhanced so that the rotational torque fromthe state Q34 to the state Q4 can be increased in the same way as hasbeen described for the states Q1, Q2 and Q3, as per the above-describedcase.

Next, as shown in FIG. 5D, energization to the coil 12 is stopped tocancel the excitation of the stator 10 to go to a state Q41. This stateQ41 is the same as the state Q23 shown in FIG. 4D except for thedifference in the polarities of the stator 20 and the rotor 30.Therefore, the magnetostatic stability is enhanced so that therotational torque from the state Q4 to the state Q41 can be increased inthe same way as has been described for the state Q23.

Further, as the current I1 is let to flow between the terminals V1 andV2 of the coil 12 while the excitation state of the stator 20 ismaintained, the state returns to the state Q1 shown in FIG. 4A. Thetransition to the state Q1 from the state Q41 is substantially the sameas the aforementioned transition to the state Q2 from the state Q12 andthe aforementioned transition to the state Q3 from the state Q23, andthe rotational torque from the state Q41 to the state Q1 can beincreased.

The above description has been given of the example where the motor 1has two auxiliary yokes 40 and 50. Depending on a case, however, one ofthe auxiliary yokes 40 and 50 may not be needed. The description of thisexample has been given of the example where the spread angles γ1 and γ2of the auxiliary yokes 40 and 50 with regard to the rotational directionof the rotor 30 is 3/2 times the spread angle α of the magnetic poles Nrand Sr of the rotor 30. However, it may be smaller than 3/2 times aslong as the angles γ1 and γ2>α. On the contrary, it may be greater than3/2 times unless the magnetic poles are not magnetostatically coupleddirectly to the magnetic poles of the adjoining stator. Here, withregard to the auxiliary yokes 40 and 50, substantially apart from themagnetic poles 13, 14, 23 and 24 of the stators means substantiallyisolation in the excited states of the magnetic poles 13, 14, 23 and 24of the stators.

Further, also the spread angles of the auxiliary yokes 40 and 50 areidentical in this example, both may be different from each other.

In addition, while the partly cylindrical auxiliary yokes 40 and 50 areillustrated as having longer axial-directional lengths than the rotor 30in the perspective view of FIG. 1, they may be shorter as long asleakage of the line of magnetic force in the axial direction can besufficiently suppressed. For example, they may be about the same as theheight (thickness) of the rotor 30 in the axial direction. That is, forexample, the upper end of the auxiliary yoke 50 may be positioned nearlyflush with a top surface 33 of the rotor 30 as indicated by an imaginaryline 50 i in FIG. 1 (the same is true of the lower end).

Second Embodiment

Further, instead of the auxiliary yoke 40 or 50 having a uniformthickness along the circumferential direction A as long as thoseportions which face both different magnetic pole portions Nr and Sr ofthe rotor 30 are substantially magnetostatically coupled, it may have athin portion 43 or 53 having a groove or a cutaway in the middle betweenboth end portions 41 and 42 or 51 and 52 in the circumferentialdirection A as shown in FIGS. 6A and 6B. An opening 44 or 54 may beformed in the middle portion in the circumferential direction A and bothend portions 41 and 42 or 51 and 52 in the circumferential direction Amay be linked at thick portions 45 or 55 on both axial-directional sidesof the opening 44 or 54 as shown in FIGS. 6C and 6D. The formation ofthe groves, the cutaways, or the openings 44 and 45 in the auxiliaryyokes 40 and 50 this way can make the stepping motor 1 lighter.

Third Embodiment

Although the foregoing description has been given of the example wherethe rotor 30 has ten poles, the number of poles of the rotor 30 may begreater or smaller.

Next, a motor 1 a whose rotor has twelve magnetic poles at equiangulardistances will be described based on FIGS. 7 and 8. Same referencenumerals are given to those members or elements of the motor 1 a inFIGS. 7 and 8 which are similar to those of the motor 1 as shown inFIGS. 1 to 6, and those members or elements which are different have asubscript “a” attached to their reference numerals.

This rotor 30 a is constructed in the same way as the rotor 30 exceptfor the number of magnetic poles being twelve. That is, the rotor 30 ahas six N poles Nr₁ to Nr₆ (denoted by reference numeral Nr when theyare not distinguished from one another or are generically referred to),and six S poles Sr₁ to Sr₆ (denoted by reference numeral Sr when theyare not distinguished from one another or are generically referred to),with the spread angle α between the individual magnetic poles being 30degrees (360 degrees/12).

Stators 10 a and 20 a of the motor 1 a are constructed in the same wayas the stators 10 and 20 except that arrangement is made in such a waythat the angle defined between the magnetic poles 14 and 24 is β2=0 andthe angle defined between the magnetic poles 13 and 23 is β1=5α. Thatis, the magnetic pole interval βs₁=βs₂=5/2α andβs₁₁=βs₁₂=βS₂₁=βs₂₂=βs=α/2 in the motor 1 a are the same as those of themotor 1 (those reference numerals are not shown in FIGS. 7 and 8 for thesake of diagrammatic simplicity). In this example, however, α=30 degreesas mentioned above.

That auxiliary yokes 40 a and 50 a are apart from the magnetic poles ofthe stators 10 a and 20 a by α/2 and spread over the angular range of3α/2 is the same as that in the case of the motor 1.

As the auxiliary yoke 40 a spreads over a larger angular range even inthe motor 1 a, the auxiliary yoke 40 a always faces the adjoiningmagnetic poles Nr and Sr of the rotor 30 a at the same time.Accordingly, a magnetic path with a low magnetic resistance is given atthe portion corresponding to the magnetic path between the magneticpoles 13 and 14 of the stator 10 a, so that the intensity of themagnetic field in the gap G₁₁, G₁₂ between the magnetic pole Nr or Srwhich faces the magnetic pole 13 and the magnetic pole Sr or Nr whichfaces the magnetic pole 14 can be enhanced. The auxiliary yoke 50 aoperates the same way. That is, as the auxiliary yoke 50 a spreads overa larger angular range, the auxiliary yoke 50 a always faces theadjoining magnetic poles Nr and Sr of the rotor 30 a at the same time,so that a magnetic path with a low magnetic resistance is given at theportion corresponding to the magnetic path between the magnetic poles 23and 24 of the stator 20 a, so that the intensity of the magnetic fieldin the gap G₂₁, G₂₂ between the magnetic pole Nr or Sr which faces themagnetic pole 23 and the magnetic pole Sr or Nr which faces the magneticpole 24 can be enhanced. Even in this case, the auxiliary yokes 40 and50 work to enhance the magnetization of the rotor. In the motor 1 a,therefore, the rotational torque with respect to the rotor 30 a can beincreased as compared with the case where there are no auxiliary yokes40 a and 50 a as has been described for the motor 1 referring to FIG. 3or FIGS. 4 and 5. The multipolarization of the rotor 30 a this way canimprove the rotational position accuracy.

Fourth Embodiment

The auxiliary yoke may be provided either between the magnetic poles ofthe stator 10 a or between the magnetic poles of the stator 20 a.

FIGS. 9 and 10 show, as a motor 1 b, an example where the auxiliaryyokes are provided between the stator magnetic poles and a partlycylindrical auxiliary yoke 5 of a soft magnetic material is providedbetween the stator 10 a and the stator 20 a.

For the sake of description, in FIG. 10, a fixed X-Y orthogonalcoordinate system is taken for the stators 10 a and 20 a of the motor 1b, and the upper direction along a mirror symmetrical center line Mc inthe plane in the diagram is taken as the positive direction of the Yaxis while the rightward direction is taken as the positive direction ofthe X axis.

The auxiliary yoke 5 is provided between the stator 10 a and the stator20 a and spreads in an angular range of γm=4α. The auxiliary yoke 5faces the circumferential surface 31 of the rotor 30 a via a narrow gapg3, and is positioned at its both ends with a gap of α/2 with respect tothe adjoining magnetic poles 13 and 23. The auxiliary yoke 5 typicallyspreads on both sides with the mirror symmetrical center line Mc as thecenter, and it is normally preferable that the spread angle (length) γmin the circumferential direction A should be larger as long as themagnetostatic coupling between the adjoining magnetic poles 13 and 23does not occur in the normal operation.

In the motor 1, as the stators 10 a and 20 a and the auxiliary yokes 40a and 50 a are actually positioned more in the −Y direction than therotor 30 a, the −Y-directional force is applied to the rotor 30 a,thereby suppressing the rotation of the rotor 30 a. In the motor 1 b, byway of contrast, the auxiliary yoke 5 is positioned facing thecircumferential surface 31 of the rotor 30 a in a substantial arealocated more in the +Y direction than the center axial line C of therotor 30 a. the auxiliary yoke 5 magnetized by the magnetic poles of therotor 30 a applies +Y-directional force to the rotor 30 a, and this+Y-directional force to the rotor 30 a, and this +Y-directionaleccentric force is balanced with the −Y-directional eccentric force tobe applied to the rotor 30 a by the auxiliary yokes 40 a and 50 a, sothat the rotor 30 a is stably positioned on the rotational center axialline C and the rotor 30 a rotates smoothly. As a result, the torqueincreased by the auxiliary yokes 40 a and 50 a laid out between themagnetic poles of the stators 10 a and 20 a help effectively andsmoothly rotate the rotor 30 a.

Fifth Embodiment

In the motor of the present invention, the number of stators may begreater than two in which case the number of the auxiliary yokes may beone, two or greater, and an auxiliary yoke is provided between themagnetic poles of each stator. As long as each auxiliary yoke is greaterthan the magnetic pole spread angle α (or its correspondingcircumferential length) of the rotor, the interval between the magneticpoles of the stator (the angular distance between the magnetic poles ofthe stator or the number of rotor magnetic poles included in themagnetic poles of the stator) may be increased and a plurality ofmagnetostatically independent auxiliary yokes may be provided betweenthe magnetic poles of the stator. In addition to the auxiliary yokes,soft magnetic material members whose spread angle is equal to or lowerthan the magnetic pole magnetic pole spread angle α of the rotor andwhich do not work as auxiliary yokes so-called in the present inventionmay be arranged at a part of the circumferential direction A facing thecircumferential surface of the rotor.

FIGS. 11 and 12 show a motor 1 c which has a rotor 30 b having fourteenmagnetic poles, four stators 10 b, 20 b, 60 and 70, and four auxiliaryyokes 40 b, 50 b, 80 and 90.

The stators 10 b and 20 b are arranged in an angular range of 5α/2 withrespect to the rotor 30 b with the fourteen poles. They are constructedin the same way as the stators 10 a and 20 a of the motor 1 a or themotor 1 b, except for that point. The entire stator 10 b and 20 b spreadover an angular range of nearly 180 degrees. The auxiliary yokes 40 band 50 b are arranged in an angular range of 5α/2 with respect to therotor 30 b with the fourteen poles.

The stator 60 is constructed in the same way as the stator 10 b or 20 bexcept for the difference in the position in the circumferentialdirection where it is provided, and has a yoke 61 of a soft magneticmaterial having a U shape (or a C shape) or a similar shape, and anexcitation coil 62. The stator 70 is constructed in the same way as thestator 60. Consequently, the stators 10 b, 20 b, 60 and 70 are rotationsymmetrical at positions around the center axial line C shifted from oneanother by 90 degrees.

The relative position of the auxiliary yoke 80 to the stator 60 issubstantially the same as the relative position of the auxiliary yoke 40b to the stator 10 b or the relative position of the auxiliary yoke 50 bto the stator 20 b, and the shape of the auxiliary yoke 80 issubstantially the same as the shape of the auxiliary yoke 40 b or theshape of the auxiliary yoke 50 b. Likewise, the relative position of theauxiliary yoke 90 to the stator 70 is substantially the same as therelative position of the auxiliary yoke 40 b to the stator 10 b or therelative position of the auxiliary yoke 50 b to the stator 20 b, and theshape of the auxiliary yoke 90 is also substantially the same as theshape of the auxiliary yoke 40 b or the shape of the auxiliary yoke 50b. Consequently, the auxiliary yokes 40 b, 50 b, 80 and 90 are rotationsymmetrical at positions around the center axial line C shifted from oneanother by 90 degrees.

Even in the motor 1 c constructed in the above-described manner, theauxiliary yokes 40 b, 50 b, 80 and 90 reduce the magnetic resistances ofthe magnetostatic paths between the magnetic poles of the correspondingstators 10 b, 20 b, 60 and 70, thereby increasing the rotational torquewith respect to the rotor 30 b.

The present invention is not limited to a stepping motor but can beadapted to an electromagnetic actuator having a stator and a multipolerotor without departing from the technical scope.

The present invention is based on Japanese Patent Application No.2003-306240 filed on Aug. 29, 2003. The present specification includesthe specification, claims, and drawings of the application by reference.

INDUSTRIAL APPLICABILITY

The electromagnetic actuator according to the present invention can beused in, for example, the drive unit of a portable device.

1. An electromagnetic actuator, comprising: a rotor (30, 30 a, 30 b)which has multiple magnetic poles (Nr, Sr) on a circumferential surface(31) thereof, and a plurality of stators (10, 10 a, 10 b, 20, 20 a, 20b, 60, 70) in an electromagnet form which have at least two magneticpoles (13, 14, 23, 24, 63, 64, 73, 74) facing said circumferentialsurface (31) of said rotor (30, 30 a, 30 b), wherein each stator in saidplurality of stators (10, 10 a, 10 b, 20, 20 a, 20 b, 60, 70)individually includes a respectively associated yoke (11, 21, 61, 71)and coil (12, 22, 62, 72), wherein each coil is configured to magnetizeits respectively associated yoke, an auxiliaiy yoke (40, 40 a, 40 b, 50,50 a, 50 b, 80, 90) extending in a circumferential direction (A) of themagnetic poles (Nr, Sr) of said rotor (30, 30 a, 30 b) over an angularrange which is larger than a spread angle (α) of said magnetic poles(Nr, Sr) of said rotor (30, 30 a, 30 b) in said circumferentialdirection (A) is provided between said magnetic poles (13, 14, 23, 24,63, 64, 73, 74) of at least one of said stators (10, 10 a, 10 b, 20, 20a, 20 b, 60, 70), wherein said auxiliary yoke (40, 40 a, 40 b, 50, 50 a,50 b, 80, 90) is not directly magnetized by said coil (12, 22, 62, 72).2. The electromagnetic actuator according to claim 1, wherein saidauxiliary yoke (40, 40 a, 40 b, 50, 50 a, 50 b, 80, 90) extending insaid circumferential direction (A) over an angular range which is largerthan the spread angle (α) of the magnetic poles (Nr, Sr) of said rotor(30, 30 a, 30 b) in the circumferential direction (A) is providedbetween said magnetic poles (13, 14, 23, 24, 63, 64, 73, 74) of each ofsaid stators (10, 10 a, 10 b, 20, 20 a, 20 b, 60, 70).
 3. Anelectromagnetic actuator, comprising: a rotor (30, 30 a, 30 b) which hasmultiple magnetic poles (Nr, Sr) on a circumferential surface (31)thereof, and a plurality of stators (10, 10 a, 10 b, 20, 20 a, 20 b, 60,70) in an electromagnet form which have at least two magnetic poles (13,14,23, 24, 63, 64, 73, 74) facing said circumferential surface (31) ofsaid rotor (30, 30 a, 30 b), wherein each stator in said plurality ofstators (10, 10 a, 10 b, 20, 20 a, 20 b, 60, 70) individually includes arespectively associated yoke (11, 21, 61, 71) and coil (12, 22, 62, 72),wherein each coil is configured to magnetize its respectively associatedyoke, an auxiliary yoke (40, 40 a, 40 b, 50, 50 a, 50 b, 80, 90)extending in a circumferential direction (A) of the magnetic poles (Nr,Sr) of said rotor (30, 30 a, 30 b) over an angular range which is largerthan a spread angle (α) of said magnetic poles (Nr, Sr) of said rotor(30, 30 a, 30 b) in said circumferential direction (A) is providedbetween said magnetic poles (13, 14, 23, 24, 63, 64, 73, 74) of at leastone of said stators (10, 10 a, 10 b, 20, 20 a, 20 b, 60, 70), whereinsaid auxiliary yoke (40, 40 a, 40 b, 50, 50 a, 50 b, 80, 90) is notdirectly magnetized by said coil (12, 22, 62, 72), wherein saidauxiliary yoke (40, 40 a, 40 b, 50, 50 a, 50 b, 80, 90) extending insaid circumferential direction (A) over an angular range which is largerthan the spread angle (α) of the magnetic poles (Nr, Sr) of said rotor(30, 30 a, 30 b) in the circumferential direction (A) is providedbetween said magnetic poles (13, 14, 23, 24, 63, 64, 73, 74) of each ofsaid stators (10, 10 a, 10 b, 20, 20 a, 20 b, 60, 70), wherein anauxiliary yoke (5) extending in said circumferential direction (A) overan angular range which is larger than the spread angle (α) of themagnetic poles (Nr, Sr) of said rotor (30, 30 a, 30 b) in thecircumferential direction (A) is provided between one stator (10 a) insaid plurality of stators (10, 10 a, 10 b, 20, 20 b, 60, 70) and anotherstator (20 a) adjoining that stator (10 a), wherein a groove, a cutawayor an opening (44, 54) is formed in an intermediate portion of themagnetic poles (Nr, Sr) of said rotor (30, 30 a, 30 b) of said auxiliaryyoke (5, 40, 40 a, 40 b, 50, 50 a, 50 b, 80, 90) in the circumferentialdirection (A).
 4. The electromagnetic actuator according to claim 1,wherein a groove, a cutaway or an opening (44, 54) is formed in anintermediate portion of the magnetic poles (Nr, Sr) of said rotor (30,30 a, 30 b) of said auxiliary yoke (5, 40, 40 a, 40 b, 50, 50 a, 50 b,80, 90) in the circumferential direction (A).
 5. The electromagneticactuator according to claim 2, wherein a groove, a cutaway or an opening(44, 54) is formed in an intermediate portion of the magnetic poles (Nr,Sr) of said rotor (30, 30 a, 30 b) of said auxiliary yoke (5, 40, 40 a,40 b, 50, 50 a, 50 b, 80, 90) in the circumferential direction (A). 6.The electromagnetic actuator according to claim 1, wherein an auxiliaryyoke (5) extending in said circumferential direction (A) over an angularrange which is larger than the spread angle (α) of the magnetic poles(Nr, Sr) of said rotor (30, 30 a, 30 b) in the circumferential direction(A) is provided between one stator (10 a) in said plurality of stators(10, 10 a, 10 b, 20, 20 a, 20 b, 60, 70) and another stator (20 a)adjoining that stator (10 a).
 7. The electromagnetic actuator accordingto claim 2, wherein an auxiliary yoke (5) extending in saidcircumferential direction (A) over an angular range which is larger thanthe spread angle (α) of the magnetic poles (Nr, Sr) of said rotor (30,30 a, 30 b) in the circumferential direction (A) is provided between onestator (10 a) in said plurality of stators (10, 10 a, 10 b, 20, 20 a, 20b, 60, 70) and another stator (20 a) adjoining that stator (10 a). 8.The electromagnetic actuator according to claim 6, wherein a groove, acutaway or an opening (44, 54) is formed in an intermediate portion ofthe magnetic poles (Nr, Sr) of said rotor (30, 30 a, 30 b) of saidauxiliary yoke (55, 40, 40 a, 40 b, 50, 50 a, 50 b, 80, 90) in thecircumferential direction (A).